Re: [PATCH v6 net-next 10/14] tcp: accecn: retransmit SYN/ACK without AccECN option or non-AccECN SYN/ACK

Paolo Abeni
2025-11-18T13:59:05Z

On 11/14/25 8:13 AM, chia-yu.chang@nokia-bell-labs.com wrote:
> From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
>
> For Accurate ECN, the first SYN/ACK sent by the TCP server shall set the
> ACE flag (see Table 1 of RFC9768) and the AccECN option to complete the
> capability negotiation. However, if the TCP server needs to retransmit such
> a SYN/ACK (for example, because it did not receive an ACK acknowledging its
> SYN/ACK, or received a second SYN requesting AccECN support), the TCP server
> retransmits the SYN/ACK without the AccECN option. This is because the
> SYN/ACK may be lost due to congestion, or a middlebox may block the AccECN
> option. Furthermore, if this retransmission also times out, to expedite
> connection establishment, the TCP server should retransmit the SYN/ACK with
> (AE,CWR,ECE) = (0,0,0) and without the AccECN option, while maintaining
> AccECN feedback mode.
>
> This complies with Section 3.2.3.2.2 of the AccECN specification (RFC9768).
>
> Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
>
> ---
> v6:
> - Use new synack_type TCP_SYNACK_RETRANS and num_retrans.
> ---
> include/net/tcp_ecn.h | 20 ++++++++++++++------
> net/ipv4/tcp_output.c | 4 ++--
> 2 files changed, 16 insertions(+), 8 deletions(-)
>
> diff --git a/include/net/tcp_ecn.h b/include/net/tcp_ecn.h
> index a709fb1756eb..57841dfa6705 100644
> --- a/include/net/tcp_ecn.h
> +++ b/include/net/tcp_ecn.h
> @@ -649,12 +649,20 @@ static inline void tcp_ecn_clear_syn(struct sock *sk, struct sk_buff *skb)
> }
>
> static inline void
> -tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th)
> -{
> - if (tcp_rsk(req)->accecn_ok)
> - tcp_accecn_echo_syn_ect(th, tcp_rsk(req)->syn_ect_rcv);
> - else if (inet_rsk(req)->ecn_ok)
> - th->ece = 1;
> +tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th,
> + enum tcp_synack_type synack_type)
> +{
> + // num_retrans will be incresaed after tcp_ecn_make_synack()

Please use /* */ for comments

> + if (!req->num_retrans) {

It's unclear you this function receives a `synack_type` argument and
don't use it. Should the above be

if (synack_type != TCP_SYNACK_RETRANS) {

?

Or just remove such argument.

/P